What Does ICD-10 Code G40.109 Mean?
ICD-10 code G40.109 describes localization-related (focal) (partial) symptomatic epilepsy and epileptic syndromes with simple partial seizures, confirmed as not intractable and occurring without status epilepticus. It is a fully billable diagnosis code valid for fiscal year 2026 (October 1, 2025 through September 30, 2026) under ICD-10-CM Official Coding Guidelines published by CMS and NCHS.
Key attributes of this code at a glance:
- Billable/valid: Yes — for all HIPAA-covered transactions in FY2026
- Applicable setting: Inpatient and outpatient
- Seizure type: Simple partial (focal aware) — consciousness is preserved
- Intractability status: Not intractable (responsive to antiepileptic drug therapy)
- Status epilepticus: Absent — the “9” in the final digit position explicitly encodes this
- Synonym used in practice: Simple partial temporal lobe epilepsy NOS
What Conditions and Diagnoses Does G40.109 Cover?
G40.109 captures focal epilepsy cases where an underlying structural or metabolic cause is suspected or confirmed, seizures remain under reasonable pharmacological control, and the patient does not experience prolonged or continuous seizure activity.
Clinical presentations appropriately reported under this code include:
- Focal motor seizures (e.g., rhythmic twitching of one hand or face) with preserved awareness
- Focal sensory seizures (e.g., paresthesias, visual phenomena, olfactory auras) without loss of consciousness
- Autonomic focal seizures (e.g., epigastric rising sensation, pallor, sweating) in isolation
- Simple partial temporal lobe epilepsy not further specified (NOS)
- Focal seizures secondary to a known brain lesion (tumor, cortical dysplasia, prior stroke) that remain drug-responsive
What Does G40.109 Specifically Exclude?
The following conditions must never be reported using G40.109. These exclusions are coded separately per ICD-10-CM tabular instruction:
- Conversion disorder with seizures → F44.5
- Convulsions NOS → R56.9 (use when epilepsy is not confirmed)
- Post-traumatic seizures → R56.1
- Seizure of newborn → P90
- Hippocampal sclerosis / mesial temporal sclerosis → G93.81 (code additionally, not instead)
- Todd’s paralysis → G83.84 (reportable as an additional code when present post-ictally)
When Is G40.109 the Right Code to Use?
Selecting G40.109 requires satisfying all three definitional axes embedded in the code structure: etiology (symptomatic), seizure type (simple partial), and clinical status (not intractable, no status epilepticus). Use the following checklist before assigning this code:
- Confirm the epilepsy is localization-related (focal/partial) — the seizure onset must be documented as arising from a discrete brain region, supported by clinical history and ideally EEG findings.
- Confirm the etiology is symptomatic, not idiopathic — “symptomatic” means there is a presumed or identified structural, metabolic, or other cause. If the cause is idiopathic (no identifiable cause, often genetic), the correct code family is G40.0x, not G40.1x.
- Confirm the seizure type is simple partial — consciousness must be preserved throughout the episode. If the patient experiences any alteration of awareness or responsiveness, the seizure has progressed to complex partial territory and G40.20x applies instead.
- Confirm the epilepsy is not intractable — the provider must NOT document terms such as pharmacoresistant, treatment resistant, refractory, or poorly controlled. Any of these terms shift the code to G40.11x.
- Confirm no status epilepticus is present — status epilepticus requires G40.101 (not intractable, with status epilepticus). The “9” in the sixth position of G40.109 is your explicit indicator that status epilepticus is absent.
How Does G40.109 Differ From Its Most Commonly Confused Codes?
| Code | Key Distinction | When to Use Instead |
|---|---|---|
| G40.109 | Symptomatic, simple partial, not intractable, no SE | This code — baseline focal epilepsy under control |
| G40.101 | Symptomatic, simple partial, not intractable, WITH status epilepticus | Provider documents SE concurrent with this episode |
| G40.119 | Symptomatic, simple partial, intractable, no SE | Provider documents refractory/pharmacoresistant/poorly controlled |
| G40.009 | Idiopathic focal epilepsy, not intractable, no SE | No underlying cause identified; presumed genetic origin |
| G40.209 | Symptomatic, complex partial seizures, not intractable, no SE | Awareness is impaired during the seizure event |
| R56.9 | Seizure NOS — not epilepsy | Single or uncharacterized seizure; epilepsy not yet confirmed |
What Documentation Is Required to Support G40.109?
Documentation is the make-or-break factor for this code. In practice, coders frequently encounter chart notes that describe focal seizures adequately but fail to explicitly address intractability status — leaving a critical axis of the code unresolved and triggering payer denials or audit flags.
What Must the Provider Document in the Clinical Notes?
The treating neurologist or provider must explicitly address each of the following in the medical record:
- Confirmed diagnosis of epilepsy — not “rule out seizure” or “history of seizure.” CMS requires an established diagnosis to use a G40 code.
- Seizure type clearly described — “simple partial,” “focal aware,” or equivalent language indicating preserved consciousness.
- Symptomatic etiology stated or implied — documentation of a causative brain lesion, prior stroke, cortical abnormality, or structural finding on imaging supports the “symptomatic” axis.
- Intractability status addressed — the provider should explicitly note that seizures are “controlled on current medication regimen” or “responding to levetiracetam.” If intractability is not addressed, coders default to “not intractable” per ICD-10-CM Official Coding Guidelines Section I.C.6 — but the absence of documentation creates audit vulnerability.
- Absence of status epilepticus confirmed — a simple note such as “no prolonged seizure activity” or “no SE” satisfies this requirement.
- Frequency and current treatment — medication name, dose, and seizure frequency support medical necessity for ongoing neurological management.
Which Diagnostic or Lab Results Support G40.109?
Supporting diagnostic findings strengthen the code’s defensibility in audits:
- EEG (electroencephalogram) — focal epileptiform discharges (e.g., sharp waves at C3/C4, temporal spike-wave) consistent with simple partial seizures; CPT 95819 (routine EEG) or 95951 (video EEG monitoring)
- MRI brain — structural abnormality in the seizure-onset zone (e.g., cortical dysplasia, cavernoma, prior infarct) supporting symptomatic classification
- PET or SPECT imaging — functional hypometabolism in the focal region, less common but supportive in pre-surgical evaluation
- Antiepileptic drug (AED) serum levels — therapeutic levels documented alongside controlled seizure activity reinforce “not intractable” status
What Is the Documentation Standard for Inpatient vs. Outpatient Settings?
| Setting | Diagnosis Standard | Coding Guidance |
|---|---|---|
| Outpatient | Code the confirmed diagnosis if established by the treating provider. Code “to the highest degree of certainty.” | Do not code unconfirmed or “suspected” epilepsy outpatient. Use R56.9 until epilepsy is confirmed. |
| Inpatient | May code conditions described as “possible” or “probable” per ICD-10-CM Official Coding Guidelines for inpatient encounters. | G40.109 can be coded if the admitting neurologist documents “probable focal symptomatic epilepsy.” |
How Does G40.109 Affect Medical Billing and Claims?
G40.109 is grouped within MS-DRG 023 (Craniotomy with major device implant or acute complex CNS principal diagnosis with MCC, or epilepsy with neurostimulator) for inpatient hospital cases meeting those thresholds. For the far more common outpatient scenario, it drives reimbursement for neurology evaluation and management, EEG interpretation, and medication management services.
Key billing considerations:
- Medical necessity: The diagnosis must support the level and type of service billed. G40.109 supports E&M codes (99202–99215), neurology consultations (99241–99245), and EEG interpretation services.
- AED monitoring: Drug level monitoring (e.g., CPT 80156 for carbamazepine, 80177 for levetiracetam) is medically necessary and directly supported by G40.109 as the linked diagnosis.
- Payer scrutiny: Medicare and many commercial payers flag epilepsy claims lacking supporting EEG or imaging documentation on file. Ensure the record contains objective diagnostic evidence before submitting.
What CPT or Procedure Codes Are Commonly Billed With G40.109?
| CPT Code | Description | Typical Pairing Context |
|---|---|---|
| 95819 | EEG, awake and asleep, routine | Initial diagnostic workup or follow-up monitoring |
| 95951 | Video EEG monitoring, 24+ hours | Presurgical evaluation or seizure characterization |
| 80177 | Levetiracetam drug level | Therapeutic AED monitoring |
| 80156 | Carbamazepine drug level | Therapeutic AED monitoring |
| 95829 | Intraoperative EEG | Surgical resection with electrocorticography |
| 99214–99215 | Office E&M, moderate-high complexity | Ongoing outpatient neurology management |
Are There Any Prior Authorization or Coverage Restrictions?
- Video EEG (95951): Most major payers require prior authorization for inpatient video EEG monitoring. Documentation must support medical necessity (e.g., failure of two or more AEDs, surgical candidacy evaluation).
- Long-term EEG (95953): Similarly subject to prior auth and requires clear documentation of the clinical question being addressed.
- Vagus nerve stimulator (VNS): Not typically covered by Medicare for G40.109 (not intractable) — payers generally require a diagnosis of intractable epilepsy (G40.119 or equivalent) before approving implantable neuromodulation devices.
- Ketogenic diet consultation: Some Medicare Advantage plans cover this for pediatric epilepsy but require medical necessity documentation linked to an established epilepsy diagnosis.
What Coding Errors Should You Avoid With G40.109?
The G40.1x subcategory is a frequent source of both under-coding and over-coding errors. Auditors reviewing epilepsy claims for this code family look for a predictable set of misapplications:
- Using G40.109 when the provider documents intractability — any mention of “refractory,” “pharmacoresistant,” “treatment resistant,” or “poorly controlled” mandates G40.119, not G40.109. This is the most common upgrade error in outpatient neurology coding.
- Using G40.109 for a single seizure or first-time seizure event — G40 codes require an established epilepsy diagnosis. First-time or unconfirmed seizures belong under R56.9 until epilepsy is diagnosed.
- Confusing simple partial with complex partial seizures — if the provider’s note describes any loss of awareness, automatisms (lip-smacking, hand-wringing), or post-ictal confusion, the seizure type is complex partial and G40.209 applies.
- Failing to distinguish idiopathic (G40.0x) from symptomatic (G40.1x) — the distinction matters for reimbursement and data integrity. Symptomatic epilepsy requires evidence of an underlying cause; idiopathic does not.
- Coding status epilepticus incorrectly — if the provider documents SE, the sixth digit changes from 9 to 1. Using G40.109 when SE is documented is a definitive coding error.
What Do Auditors Look for When Reviewing Claims With G40.109?
- Missing or absent EEG findings in the medical record when an EEG CPT code is billed alongside
- No explicit provider documentation of intractability status (or lack thereof)
- Mismatched seizure type between clinical notes and coded diagnosis
- Use of G40.109 when the provider narrative clearly describes loss of awareness (should be G40.209)
- AED serum level orders without a supporting epilepsy diagnosis tying the test to medical necessity
- VNS or ketogenic diet authorization requests submitted with a “not intractable” diagnosis code
How Does G40.109 Relate to Other ICD-10 Codes?
G40.109 sits within the G40.1x subcategory of focal (localization-related) symptomatic epilepsy with simple partial seizures. Understanding its relationships to adjacent codes prevents both under-coding and claim denials.
| Code | Relationship to G40.109 | Key Distinction |
|---|---|---|
| G40.101 | Same subcategory — different SE axis | Identical except WITH status epilepticus |
| G40.119 | Same subcategory — intractability upgrade | Seizures are pharmacoresistant/refractory |
| G40.009 | Sibling code — idiopathic etiology | No identifiable structural or metabolic cause |
| G40.209 | Adjacent subcategory — complex partial seizures | Consciousness is impaired during the event |
| G93.81 | Use-additional code when present | Hippocampal/mesial temporal sclerosis as underlying cause |
| G83.84 | Use-additional code when applicable | Todd’s paralysis occurring post-ictally |
| R56.9 | Alternate — not epilepsy | Use for unconfirmed or single seizure events |
| Z82.61 | Additional code | Family history of epilepsy (supporting documentation) |
What Is the Correct Code Sequencing When G40.109 Appears With Other Diagnoses?
- G40.109 as principal diagnosis — appropriate when the epilepsy is the primary reason for the encounter (e.g., outpatient neurology visit for seizure management).
- Underlying cause coded additionally — if hippocampal sclerosis (G93.81) or another structural etiology is documented, report it as an additional diagnosis after G40.109.
- Todd’s paralysis (G83.84) — add as an additional code when the provider documents post-ictal focal weakness or paralysis following the seizure event.
- Medication management — if the encounter also addresses AED management, Z79.899 (long-term use of other medication) may be appropriate as an additional code depending on payer policy and encounter context.
- Do not code R56.9 alongside G40.109 — they are mutually exclusive. R56.9 is for unspecified seizures without an established epilepsy diagnosis.
Real-World Coding Scenario — How G40.109 Is Applied in Practice
Patient Encounter: A 34-year-old patient presents to neurology for a follow-up visit. The patient has a documented history of right temporal lobe epilepsy following resection of a cavernous malformation two years ago. The provider notes: “Patient continues to experience focal aware seizures approximately twice monthly, characterized by a rising epigastric sensation and right hand tingling, lasting 30–60 seconds. No loss of consciousness. Seizures occur despite levetiracetam 1,500 mg BID and have remained at this baseline frequency for 12 months without worsening. No episodes of status epilepticus. Current regimen is providing reasonable, if incomplete, control. Will continue current therapy and recheck levetiracetam level.”
Correct Code Application
- G40.109 — Localization-related symptomatic epilepsy with simple partial seizures, not intractable, without status epilepticus
- Rationale: Focal (temporal) onset confirmed; symptomatic etiology documented (prior cavernous malformation); consciousness preserved during events; provider describes “reasonable control,” not refractoriness; no SE documented.
- G93.81 — Not applicable here since sclerosis is not mentioned; the malformation was previously resected.
- 80177 — Levetiracetam drug level (if ordered at this visit).
Common Mistake in This Scenario
- Incorrect code selected: G40.119 (intractable)
- Why it fails: The provider states the regimen is providing “reasonable, if incomplete, control” — this language does not meet the ICD-10-CM definition of intractability. Intractable epilepsy requires explicit provider documentation using equivalent terms: pharmacoresistant, treatment resistant, refractory, or poorly controlled. “Incomplete control” alone does not qualify. Assigning G40.119 without that explicit language constitutes an unsupported code upgrade and creates audit exposure.
Frequently Asked Questions About ICD-10 Code G40.109
Is ICD-10 Code G40.109 Valid for Use in FY2026?
G40.109 is a valid, billable ICD-10-CM diagnosis code for fiscal year 2026, covering transactions from October 1, 2025 through September 30, 2026 with no changes to its description or validity status. Coders should verify annually against the ICD-10-CM Official Coding Guidelines released by CMS to confirm the code has not been revised or replaced.
What Is the Difference Between G40.109 and G40.119?
G40.109 designates simple partial focal epilepsy that is not intractable — meaning seizures respond to antiepileptic drug therapy and the provider has not documented treatment resistance. G40.119 applies when the provider explicitly documents the epilepsy as pharmacoresistant, refractory, treatment resistant, or poorly controlled, which are all recognized equivalent terms per the ICD-10-CM tabular instruction at the G40 category level.
What Is the Difference Between G40.109 (Symptomatic) and G40.009 (Idiopathic)?
G40.109 applies when there is an identified or presumed underlying cause for the focal seizures, such as a brain lesion, cortical dysplasia, prior stroke, or other structural abnormality. G40.009 applies when no causative factor is identified and the epilepsy is presumed to be idiopathic or genetic in origin. The distinction requires clear provider documentation of the etiology, not coder inference from imaging results alone.
Can G40.109 Be Coded in an Outpatient Setting?
Yes, G40.109 may be used in outpatient settings, provided the treating provider has established a confirmed diagnosis of focal symptomatic epilepsy with simple partial seizures. Outpatient coding guidelines prohibit using G40 codes for “suspected” or “probable” epilepsy — the diagnosis must be stated as definitive. If the epilepsy is not yet confirmed, report the presenting symptom code (e.g., R56.9) until a firm diagnosis is established.
What Happens to the Code If the Patient Later Develops Intractable Epilepsy?
When a patient’s documented seizure control deteriorates and the provider explicitly describes the condition as refractory, treatment resistant, or pharmacoresistant, the correct code changes to G40.119 (not intractable to intractable, without status epilepticus). This upgrade must be driven by provider language — the coder cannot infer intractability from medication changes or seizure frequency alone without corresponding physician documentation.
Does G40.109 Support Medical Necessity for an EEG?
G40.109 generally supports medical necessity for routine and follow-up EEG studies, provided the clinical documentation articulates a clear reason for the test (e.g., change in seizure pattern, medication adjustment, new symptoms). Per CMS coverage policies, EEG orders linked to an established epilepsy diagnosis are generally reimbursable, though payers may require that the ordering provider’s notes reflect a clinically appropriate indication beyond the diagnosis code alone.
Can G40.109 and G93.81 Be Coded Together?
Yes — G93.81 (hippocampal sclerosis/mesial temporal sclerosis) may be reported as an additional code alongside G40.109 when the provider documents this structural finding as the underlying cause of the focal epilepsy. The G40.109 code remains the principal diagnosis for the epilepsy itself, and G93.81 serves as a secondary code providing etiological specificity — a practice supported by AHA Coding Clinic guidance on use-additional-code conventions.
Key Takeaways
Every coder working with epilepsy claims should keep these principles in mind when applying G40.109:
- G40.109 requires three explicitly documented axes: symptomatic etiology, simple partial (focal aware) seizure type, and not-intractable status without status epilepticus.
- The sixth digit position (9 vs. 1) encodes status epilepticus — never overlook it when reviewing the provider note.
- “Symptomatic” means a structural or metabolic cause is present or suspected; if the epilepsy is idiopathic, the correct family is G40.0x.
- Intractability is a provider-defined term — coders cannot infer it from seizure frequency, medication counts, or incomplete seizure control without explicit documentation.
- EEG results, MRI brain findings, and AED levels are the three strongest pillars of coding audit preparation for all G40.1x claims.
- VNS and surgical authorization requests tied to G40.109 will commonly be denied — intractable focal epilepsy (G40.119) is the expected diagnosis for those services.
- For further guidance, coders should consult the ICD-10-CM Official Coding Guidelines, CMS epilepsy LCD references, and peer-reviewed resources such as the AAN’s published ICD-10 epilepsy crosswalk guidance when managing complex epilepsy coding scenarios.